P. Sunderrajan

NEW DELHI: The India Meteorological Department on Friday forecast that rainfall during the coming southwest monsoon season would be near normal, at 96 per cent of the long period average (LPA).

The model error will be plus or minus 5 per cent for the country as a whole and for the season in its entirety. In other words, the rainfall could be between 91 per cent and 101 per cent of the LPA, which corresponds to the average of rainfall for the 50-year period from 1941 to 1990, is 89 cm.

The forecast is based on the same five-parameter statistical ensemble forecasting system adopted in 2008 and 2007. Last year, the forecast proved almost right. The IMD predicted a rainfall of 99 per cent of the LPA; the actual came to 98 per cent. In 2007, on the other hand, the IMD forecast 95 per cent of the LPA and the actual was 105 per cent. The five parameters are: sea surface temperature over the North Atlantic Ocean during December-January; sea surface temperature over equatorial south Indian Ocean during February-March; mean sea level pressure in East Asia during February-March; the land surface air temperature over north-western Europe in January; and warm water volume over the Equatorial Pacific Ocean in February-March. The IMD will make a second-stage forecast in June. As usual that will have three components: update of the forecast for the country as a whole and the season in its entirety; separate forecasts for July and August rainfalls for the country as a whole; and seasonal rainfalls for the four geographical regions.

Experimental forecasts

In a statement, the IMD said that as part of ongoing efforts to improve the long-term forecast capabilities, it had generated experimental forecasts based on a dynamical forecasting system and taken into account experimental forecasts by national and international institutes.

The national institutes include the Indian Institute of Tropical Meteorology, the Indian Institute of Science, the Space Application Centre, the National Aerospace Laboratories, the Centre for Mathematical Modelling and Computer Simulation and the National Centre for Medium Range Weather Forecasting.

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